You have a good question about the controls needed for this project. Here is a good explanation of variables from the Science Buddies website: http://www.sciencebuddies.org/science-f
In this experiment, your independent variable is the sports drink or orange juice that you use for testing. The dependent variable is the conductivity reading that you measure. You are correct, one control that you can use to verify that your conductivity meter is working properly is the water. If your local tap water has a high conductivity, you could use distilled water as this should have a very low conductivity.
For a positive control, you could use sterile saline from the pharmacy, which contains 150 mM sodium chloride, or you could weigh different concentrations of table salt (sodium chloride) and dissolve it in distilled water.
The following website includes a standard operating procedure for calibrating a conductivity meter and a recipe for making official conductivity standard using potassium chloride. http://www.cdpr.ca.gov/docs/emon/pubs/sops/eqwa004.pdf
Please note that it is important to making all of your conductivity measurements with samples that are at the same temperature. I recommend using a thermometer that measures degrees Centigrade and record the temperature of your samples to verify that temperature is one of your controlled parameters.
I hope this helps. Please post again if you have any questions.